In a typical fiber optic transmission network, information is conveyed in the form of optical channel signals generated by light sources such as lasers or light-emitting diodes. In order to transmit the information optically in a network, a payload signal is used to modulate the source input current of a particular light source so as to impress the optical output of that source with the content of the information to be transmitted. For example, digital information can be transposed onto an optical channel signal by modulating the associated source current between two fixed amplitude levels such that the light source produces an ON-OFF keyed (OOK) optical output operating at two distinct optical intensity levels. These different intensities in the optical output can then be used for representing binary data according to the form of encoding used.
In order to monitor the transmission of information, it is well known to administrate and maintain the optical channels by using a small portion of the available channel bandwidth in the fiber to transmit a low frequency, low amplitude overhead signal. This overhead signal is typically superimposed onto the payload information signal (hereinafter referred to as the payload signal) before the latter is applied to modulate a light source. As a result of the modulation, the light source produces an optical channel signal carrying both the payload and overhead information for transmission in a particular optical channel.
In current systems, the overhead modulation is carried out at a fixed modulation depth which is typically 5% to 10% of the payload signal amplitude. As an example, for a 20 mA payload signal the overhead signal may be set to .+-.0.5 mA (1 mA peak-to-peak) for a modulation depth of 5%. This high level of modulation depth is required to ensure proper demodulation and recovery of the overhead information at a suitable signal-to-noise ratio (SNR). However, the presence of an overhead signal modulated with a high modulation depth reduces the signal-to-noise ratio (SNR) of the payload signal and results in a substantial degradation of the system's performance. For systems which necessitate the transmission of overhead information, it would be desirable to transmit overhead signals at a lower modulation depth for improving payload SNRs and minimize the performance penalty.
In addition to the need to reduce the modulation depth at which overhead signals are modulated, it would also be desirable to have available a larger overhead bandwidth for transmitting more overhead information to improve management and control flexibility. Currently however, this need cannot be effectively addressed without resulting in a substantial penalty in performance. A larger overhead bandwidth will interfere more with the payload signal content, particularly in systems such as wavelength division multiplexed (WDM) networks for example, in which the payload signal necessitates a large bandwidth to accommodate multiple payload signals operating at different bit rates. With a larger bandwidth, the payload signal may overlap with an overhead signal, more particularly in the low frequency range where the frequency content of the overhead signal is typically located. This would cause a degradation of the payload signal SNR and seriously affect the system's performance. Therefore, it is also desirable to have an overhead signal which can carry additional information without seriously impeding the system's performance.
Presently, in order to retrieve an overhead signal from an optical channel signal, the optical channel signal must first be demodulated and converted into an electric form before any manipulation of the overhead information can be carried out. The optoelectrical conversion and demodulation of the entire optical channel signal can become quite expensive, particularly in transit terminals where only the overhead information is required for control and management. As optical technology evolves toward all optical networking, there will be a need to monitor optical signals at various points for fault detection and performance monitoring applications. A low cost, high SNR gain technique for recovering the overhead information without having to do a complete optical channel demodulation (payload and overhead) would also be desirable in the control of such networks.